6,780 research outputs found
The enigmatic young brown dwarf binary FU Tau: accretion and activity
FU Tau belongs to a rare class of young, wide brown dwarf binaries. We have
resolved the system in a Chandra X-ray observation and detected only the
primary, FU Tau A. Hard X-ray emission, presumably from a corona, is present
but, unexpectedly, we detect also a strong and unusually soft component from FU
Tau A. Its X-ray properties, so far unique among brown dwarfs, are very similar
to those of the T Tauri star TW Hya. The analogy with TW Hya suggests that the
dominating soft X-ray component can be explained by emission from accretion
shocks. However, the typical free-fall velocities of a brown dwarf are too low
for an interpretation of the observed X-ray temperature as post-shock region.
On the other hand, velocities in excess of the free-fall speed are derived from
archival optical spectroscopy, and independent pieces of evidence for strong
accretion in FU Tau A are found in optical photometry. The high X-ray
luminosity of FU Tau A coincides with a high bolometric luminosity confirming
an unexplained trend among young brown dwarfs. In fact, FU Tau A is
overluminous with respect to evolutionary models while FU Tau B is on the 1 Myr
isochrone suggesting non-contemporaneous formation of the two components in the
binary. The extreme youth of FU Tau A could be responsible for its peculiar
X-ray properties, in terms of atypical magnetic activity or accretion.
Alternatively, rotation and magnetic field effects may reduce the efficiency of
convection which in turn affects the effective temperature and radius of FU Tau
A shifting its position in the HR diagram. Although there is no direct prove of
this latter scenario so far we present arguments for its plausibility.Comment: Accepted for publication in MNRAS; 9 pages, 5 figure
Evidence for Variable, Correlated X-ray and Optical/IR Extinction toward the Nearby, Pre-main Sequence Binary TWA 30
We present contemporaneous XMM-Newton X-ray and ground-based optical/near-IR
spectroscopic observations of the nearby ( pc), low-mass (mid-M)
binary system TWA 30A and 30B. The components of this wide (separation
3400 AU) binary are notable for their nearly edge-on disk viewing
geometries, high levels of variability, and evidence for collimated stellar
outflows. We obtained XMM-Newton X-ray observations of TWA 30A and 30B in 2011
June and July, accompanied (respectively) by IRTF SpeX (near-IR) and VLT
XSHOOTER (visible/near-IR) spectroscopy obtained within 20 hours of the
X-ray observations. TWA 30A was detected in both XMM-Newton observations at
relatively faint intrinsic X-ray luminosities (
) compared to stars of similar mass and age . The intrinsic
(0.15-2.0 keV) X-ray luminosities measured in 2011 had decreased by a factor
20-100 relative to a 1990 (ROSAT) X-ray detection. TWA 30B was not detected,
and we infer an upper limit of ( 3.0 erg
s). We measured a large change in visual extinction toward TWA 30A (from
to ) between the two 2011 observing epochs,
and we find evidence for a corresponding significant decrease in X-ray
absorbing column (). The apparent correlated change in and is
suggestive of variable obscuration of the stellar photosphere by disk material
composed of both gas and dust. However, in both observations, the inferred
to ratio is lower than that typical of the ISM, suggesting that
the disk is either depleted of gas or is deficient in metals in the gas phase.Comment: 10 pages, 7 figures, Accepted for publication in MNRA
X-ray observations of IC348 in light of an updated cluster census
IC348 is an excellent laboratory for studies of low-mass star formation being
nearby, compact and rich. A Chandra observation was carried out early in the
satellite's lifetime. The extensive new data in optical and infrared
wavelengths accumulated in subsequent years have changed the cluster census
calling for a re-analysis of the X-ray data.Comment: poster paper to appear in Proc. of the 15th Workshop on Cool Stars,
Stellar Systems and the Su
The Multiple Young Stellar Objects of HBC 515: An X-ray and Millimeter-wave Imaging Study in (Pre-main Sequence) Diversity
We present Chandra X-ray Observatory and Submillimeter Array (SMA) imaging of
HBC 515, a system consisting of multiple young stellar objects (YSOs). The five
members of HBC 515 represent a remarkably diverse array of YSOs, ranging from
the low-mass Class I/II protostar HBC 515B, through Class II and transition
disk objects (HBC 515D and C, respectively), to the "diskless", intermediate-
mass, pre-main sequence binary HBC 515A. Our Chandra/ACIS imaging establishes
that all five components are X-ray sources, with HBC 515A - a
subarcsecond-separation binary that is partially resolved by Chandra - being
the dominant X-ray source. We detect an X-ray flare associated with HBC 515B.
In the SMA imaging, HBC 515B is detected as a strong 1.3 mm continuum emission
source; a second, weaker mm continuum source is coincident with the position of
the transition disk object HBC 515C. These results strongly support the
protostellar nature of HBC 515B, and firmly establish HBC 515A as a member of
the rare class of relatively massive, X-ray luminous "weak-lined T Tauri stars"
that are binaries and have shed their disks at very early stages of pre-MS
evolution. The coexistence of two such disparate objects within a single,
presumably coeval multiple YSO system highlights the influence of pre- MS star
mass, binarity, and X-ray luminosity in regulating the lifetimes of
circumstellar, planet-forming disks and the timescales of star-disk
interactions.Comment: Accepted for publication in A&A; 11 pages, 5 figure
Numerical study of surface-induced reorientation and smectic layering in a nematic liquid crystal
Surface-induced profiles of both nematic and smectic order parameters in a
nematic liquid crystal, ranging from an orienting substrate to "infinity", were
evaluated numerically on base of an extended Landau theory. In order to obtain
a smooth behavior of the solutions at "infinity" a boundary energy functional
was derived by linearizing the Landau energy around its equilibrium solutions.
We find that the intrinsic wave number of the smectic structure, which plays
the role of a coupling between nematic and smectic order, strongly influences
the director reorientation. Whereas the smectic order is rapidly decaying when
moving away from the surface, the uniaxial nematic order parameter shows an
oscillatory behavior close to the substrate, accompanied by a non-zero local
biaxiality.Comment: LaTeX, 17 pages, with 4 postscript figure
Correlated decays of pair-produced scalar taus
We study the quantum mechanical correlation between two identical neutralinos
in the decays of minimal supersymmetric standard model (MSSM) scalar tau (stau)
pair produced in e+e- annihilation. Generally, the decay products of scalar
(spinless) particles are not correlated. We show that a correlation between two
neutralinos appears near pair production threshold, due to a finite stau width
and mixing of the staus and/or neutralinos, and because the neutralinos are
Majorana. Because the correlation is significant only in a specific kinematical
configuration, it can be observed only in supersymmetric models where the
neutralino momenta can be kinematically reconstructed, such as in models with
R-parity violation.Comment: revtex, 10 pages, 8 eps figures; references added, version to appear
in PR
A study of top polarization in single-top production at the LHC
This paper complements the study of single top production at the LHC aiming
to estimate the sensitivity of different observables to the magnitude of the
effective couplings. In a previous paper the dominant -gluon fusion
mechanism was considered, while here we extend the analysis to the subdominant
(10% with our set of experimental cuts) s-channel process. In order to
distinguish left from right effective couplings it is required to consider
polarized cross-sections and/or include effects. The spin of the top is
accessible only indirectly by measuring the angular distribution of its decay
products. We show that the presence of effective right-handed couplings implies
necessarily that the top is not in a pure spin state. We discuss to what extent
quantum interference terms can be neglected in the measurement and therefore
simply multiply production and decay probabilities clasically. The coarsening
involved in the measurement process makes this possible. We determine for each
process the optimal spin basis where theoretical errors are minimized and,
finally, discuss the sensitivity in the s-channel to the effective right-handed
coupling. The results presented here are all analytical and include
corrections. They are derived within the narrow width approximation for the
top.Comment: 30 pages, 14 figure
Weak boson fusion production of supersymmetric particles at the LHC
We present a complete calculation of weak boson fusion production of
colorless supersymmetric particles at the LHC, using the new matrix element
generator SUSY-MadGraph. The cross sections are small, generally at the
attobarn level, with a few notable exceptions which might provide additional
supersymmetric parameter measurements. We discuss in detail how to consistently
define supersymmetric weak couplings to preserve unitarity of weak gauge boson
scattering amplitudes to fermions, and derive sum rules for weak supersymmetric
couplings.Comment: 24 p., 3 fig., 9 tab., published in PRD; numbers in Table IV
corrected to those with kinematic cuts cite
Top Quark Spin Polarization in ep Collision
We discuss the degree of spin polarization of single top quarks produced via
fusion process in collision at TESLA+HERAp and CLIC+LHC energies
and 5.3 TeV. For subprocess we show that
the top quark spin is completely polarized when the spin basis is chosen in the
direction of the incoming positron beam in the rest frame of top quark. A
description on how to combine the cross sections of and
processes is given. -beam direction is
taken to be the favorite top quark spin decomposition axis in its rest frame
and it is found to be comparable with the ones in collision. It is argued
that theoretical simplicity and experimental clearness are the advantage of
collision.Comment: Revised version of Phys. Rev. D69 (2004)03401
The X-ray emission from Z CMa during an FUor-like outburst and the detection of its X-ray jet
Accretion shocks have been recognized as important X-ray emission mechanism
for pre-main sequence stars. Yet the X-ray properties of FUor outbursts, events
that are caused by violent accretion, have been given little attention. We have
observed the FUor object Z CMa during optical outburst and quiescence with
Chandra. No significant changes in X-ray brightness and spectral shape are
found, suggesting that the X-ray emission is of coronal nature. Due to the
binary nature of Z CMa the origin of the X-ray source is ambiguous. However,
the moderate hydrogen column density derived from our data makes it unlikely
that the embedded primary star is the X-ray source. The secondary star, which
is the FUor object, is thus responsible for both the X-ray emission and the
presently ongoing accretion outburst, which seem however to be unrelated
phenomena. The secondary is also known to drive a large outflow and jet, that
we detect here for the first time in X-rays. The distance of the X-ray emitting
outflow source to the central star is higher than in jets of low-mass stars.Comment: 5 pages, accepted for publication in Astronomy & Astrophysic
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